I'm looking for advice in setting up a scenario:

• A task is assigned to one person.
• The person has estimated it will take 20 hours to complete the task over the course of a 40-hour work week.
• The completed task actually took 30 hours to complete

My questions:

• Which fields is data entered into initially
• What values are entered into the fields initially
• How are the actuals entered

Create the task and the person. Assign the resource at 50% and enter 20 hours of work. Project will calculate 1 week duration.
Save a baseline to capture the "plan".
You can then enter 30 hours of actual work and project will mark the task 100% complete and extend the duration to allow 30 hours of work in 60 hour duration.
So, did they do the 30 hours of work within a week? If so, switch to the Task Usage view and enter the 30 hours in actual work in the time-phased portion to end after one week.